As devices for controlling steerable wheels for vehicles, there has heretofore been known a steering control device of the steer-by-wire type that a steering wheel is mechanically disconnected from a steering gear box which is in linkage to front wheels (steerable wheels). In the steering control device, the steering wheel is not mechanically connected to the steering gear box, and the steering angle of the steering wheel is detected so that the steering gear box is driven by an electric motor in dependence on the detected steering angle.
In the steering control device of this kind, a fail-safe capability of a certain type is provided so that the steering operation is enabled to be continued reliably in the event of a failure of the device. As one for performing the fail-safe capability, there has been proposed a construction having two duplex systems, as described in Japanese unexamined, published patent application No. 2002-37112.
One of the two systems operates as a main system, which is provided with one motor for driving a shaft coupled to steerable wheels, one drive circuit for driving the motor, one control circuit for controlling the motor through the drive circuit and various sensors necessary for the control. The other system operates as a secondary system, which is provided with the same construction as the main system.
In the device with the two systems, in the normal state, the respective motors are operated not to interfere with each other, and the motor for the main system operates to drive the shaft in dependence on the steering angle of the steering wheel. That is, although the two motors of the both systems are operated simultaneously, they do not cooperate to drive the shaft coupled to the steerable wheels. Further, when the main system falls in failure for some causes, the motor for the main system is made to stop, and the motor for the remaining secondary system is brought into operation to drive the shaft.
However, in the foregoing device, the motor for the main system which actually drives the shaft in the normal state and the other motor for the secondary system which does not actually drive the shaft in the normal state are designed to be of the same specification which has such a large output that is capable of driving the shaft singly in dependence on the steering angle even when the reaction force from the road surface is large. This gives rise to a problem that it is unable to reduce the cost for the motors, which causes the steering control device to be of a high cost.
Further, it may be attempted that in a different way from the technology proposed in the Japanese unexamined, published patent application No. 2002-37112, the motor for the secondary system is made to remain stopped and the switching is made to the secondary system in the event that the main system falls in failure. However, even if such an attempt were made, the difficulty in the cost reduction of the motors would remain so far as the both motors are same in the specification of their power outputs. Furthermore, as far as such plural (two or more) duplex systems are provided wherein motors for the respective systems are same in the power output specification, the similar problem in reducing the cost of the motors arises not only in the steering control device of the steer-by-wire type.
It is an object of the present invention to provide a vehicle steering control device and a vehicle steering control method which are capable of reducing the system cost irrespective of the provision of plural systems.